In projecting images using an optical projector that is configured for a compact size, various lamps, lenses, reflectors and spatial light modulators such as digital micro-mirror devices (DMD), liquid crystal display (LCD) and liquid-crystal-on-silicon (LCoS) devices are often used. The optical projection systems are formed in two parts, an illumination system for generating and collecting the light rays needed to illuminate an image, and a projection system for collecting the illuminated image light rays into a projection lens and then projecting the final image out of the compact optical device. A spatial light modulator device receives the illumination light rays from the illumination system, modulates the light received with image data, and transmits image light rays for projection to the projection system. However, in the prior known systems, substantial area is required for the illumination system, the spatial light modulator, the projection system, and the light path needed to project an image from a compact optical device.
FIG. 1 depicts in a simple illustration a typical application for a compact optical projector in the form of a smartphone 10. In FIG. 1, the smartphone 10 has a chassis or body 11 with a thickness labeled ‘T’ in FIG. 1, a touchscreen 13 for displaying information and for receiving user inputs, and a projection lens 17 for an embedded compact optical projector within smartphone 10 (the embedded compact optical projector is not visible in FIG. 1) which is shown projecting a beam of light 15.
There are several challenges associated with providing an embedded optical projection feature in a device such as smartphone 10 using prior known solutions. These portable devices are increasingly being made smaller and in particular, thinner. The thickness ‘T’ is constantly being reduced, for example, it may be less than 7 millimeters in current devices. Further challenges are presented by the overall small size of the device and by the fact that within that portable device many functions are provided that also require circuit board and system space. These functions can include, without limitation, cellular phone integrated circuits, WiFi and Bluetooth integrated circuits, various input and output features such as a compact flash card or USB card ports, SIM card sockets, front and back side cameras, speakers, microphones, audio and vibratory alert functions, various external input and output jacks, circuitry for GPS, motion detection such as accelerometers and gyroscopes, and the like. In addition the device 10 requires various processors and controllers and data storage such as static, non-volatile and dynamic memory, all of which require space within chassis 11. Further the device 10 often operates on battery power, so that the brightness from an embedded projector must be maximized for a small power budget. The projector efficiency performance metric, which can be for example lumens/Watt, must be optimized. The prior known solutions for compact optical projection systems fail to provide adequate performance in a size that is compatible with current and future portable device sizes and with available battery capacities.
In order to further illustrate the problems associated with the prior known solutions, FIG. 2 depicts in a system block diagram a top view or plan view of a prior known projection system 40. System 40 is configured using conventional reverse total internal reflection (RTIR) projection architecture. In an RTIR architecture, a reverse total internal reflection (RTIR) prism is used in a projection path between a spatial light modulator and the projection optics, as is further described below.
In the projection system 40, illumination is provided as shown as provided by the use of red, green and blue (RGB) LEDs 42, 43 and 46. However, alternative illumination sources can also be used, such as incandescent lamps with reflectors, single lamps with color wheels, laser, laser-phosphor illumination, and the like. The LEDs can include an optical coating or collimating optics 41 which act to collect and collimate the light output by the LEDs. Also, as illustrated in FIG. 2, two LEDs 42 and 46 are shown on a single integrated device, these can be the red and green LED devices, for example, while the blue LED 43 is provided as a separate component. In alternative systems three individual LEDs are used, and two dichroic plates in the form of an X shape can be used to combine the three colors (RGB) into an illumination source. In the particular example shown in FIG. 2, dichroic plate 48 reflects the light from red LED 46 at one surface, reflects the light from green LED 42 at a second surface, and passes the light from blue LED 43 through and to the illumination path. Note that in alternative arrangements, many LEDs can be used or multiple LEDs can be used instead of one LED for each color.
In FIG. 2, an additional collimator 49 for example is placed between the LEDs 42, 46 and the dichroic plate 48. Collimators are well known and perform the function of reducing the beam diverging angle. Integrator 47 is placed in the illumination path after the dichroic plate. The integrator 47 may be a “flys-eye” integrator (also referred to as a lens array), or a rod integrator or tube integrator. The integrator produces a more homogeneous light beam which can then be transmitted through one or more relay lenses such as relay 51. The relay optics such as 51 extend the length of the illumination path.
Mirror 61 is provided and in this particular example arrangement, folds the illumination light path. This reflective fold mirror also enables the illumination light rays to reach the spatial light modulator 73, which in this example is a digital micro-mirror device, at an angle. Because the digital micro-mirror (DMD) 73 modulates the light by tilting reflective mirrors, the illumination rays must strike the mirrors at an angle. Use of the folding mirror 61 makes control of the angle of the illumination rays to the spatial light modulator 73 easier to achieve. Additional relay optics such as 52 can be placed between the mirror 61 and the DMD 73.
Use of a reflective spatial light modulator such as DMD 73 requires that the illumination light rays from mirror 61 that are entering the DMD package and the reflected image light rays leaving the mirrors in spatial light modulator 73 be physically separated to avoid interference, as can be seen by examining FIG. 2. As is known in the art, the use of a RTIR prism can separate the incoming rays from the illumination system from the image rays that are being transmitted into the projection optics. U.S. Pat. No. 5,309,188, entitled “Coupling Prism Assembly and Projection System Using Same,” which is hereby incorporated by reference in its entirety herein, discloses a prism arrangement using total internal reflection to separate the illumination and projection light paths in a small space. As shown in FIG. 2, wedge prism 75 and TIR prism 76 form a coupling prism that accomplishes the needed separation of the illumination light rays from the image light rays. The image light rays exit prism 76 and are coupled into a projection system that includes elements 54, 56, and 59
FIG. 2 illustrates a space 50 between the path formed by the LED light sources and illumination optics and the path for the projection optics. This space 50 is not used as part of the projection optics but is nonetheless required for the RTIR projection system 40. The space 50 is wasted. In an application for a portable compact optical projector, such as an embedded projector in a smartphone, this use of space is undesirable.
FIG. 3 illustrates in additional detail a block diagram of the optics in a projection path 80 for a prior known system such as that in FIG. 2. As can be seen from FIG. 3, there are several optical components following the spatial light modulator 93, and prism 95, including a space followed by a large field lens 97 and various lenses up to pupil 98, and several magnification lenses and projection lenses including lens 99 following after pupil 98. Accordingly the length ‘L’ of the optics from the DMD 93 to the output is quite long, a factor that is quite disadvantageous and even prohibitive for embedded compact optical projection systems. Also, the light path followed from the DMD 93 to the output lens 99 has a large height that is larger than the height shown as ‘H’ of the spatial light modulator, as the light rays extend first upward above the height ‘H’ as the light moves towards the pupil 98, and then extend downwards as it travels from the pupil 98 outwards, dropping below the bottom of the vertical height ‘H’ (as indicated by the dashed lines in FIG. 3). In order to maintain all of the lumens available for projection and so improve or maintain efficiency, the system using the prior known projection system 80 must allow extra vertical space for this light path that travels above, and then travels below, the vertical space ‘H’. This height requirement in the known prior solutions is also disadvantageous when forming a compact embedded optical projection system, as it increases the space required in order to maintain the brightness from the LED sources throughout the projection system (to conserve the lumens output by the light sources so as to obtain the maximum available brightness at the output).
Improvements in the compact optical projection systems for embedding optical projectors in portable or small form factor devices, such as for embedding optical projectors in smartphones, are therefore needed in order to address the deficiencies and the disadvantages of the prior known approaches. Solutions are needed that reduce the total number of components; reduce the area and space required for the embedded compact optical projectors, and which improve the performance, for example in terms of performance metrics such as brightness in lumens/Watt, while maintaining or improving the image resolution of the embedded compact optical projection systems.